Before you do anything: smack the sh*t outta your brother. Then exercise and eat healthy. Those won't help for tomorrow, but they will help a lot later. The best thing is confidence. Don't let what others think affect your day in a negative way. They are just shallow ass holes if they automatically start judging you. So girl, go out there and rock that bikini, and have fun! Confidence is sexy as hell, remember that! Good Luck hun =)
First, proper fit (for your figure type) bikini can make you look better. Don't eat much before going to the beach, do couple of press exercises, in the morning apply body lotion to hydrate the skin and make it look healthy for the time when you go out. Also don't forget to smack your brother for saying you look fat in a bikini - it will give you some confidence boost =D
Don't listen to your brother, brothers have the tendency to say stuff like that. I know my brother has done the same thing. If he says anything about it again don't listen to him, he's bringing you down and you're letting him get to you.
dont sloutch, be confident, and hold you body in ways that flatter you. When laying out stretch your arms over your head and bend one knee. Also try some makeup to conture your curves and give you abs and cleavage.